S⌀ Spherical diameter
Spherical diameter · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “⌖ S⌀ 0.1 |A|B|C|” states a spherical diameter requirement with a cylindrical tolerance zone of ⌀ 0.1 mm, measured from datums A, B, C in that order of precedence.
What controls
A prefix indicating that the value following it is the diameter of a sphere, not of a cylinder. Placed on a dimension, it describes a spherical surface; placed inside a feature control frame, it turns the zone into a sphere instead of a cylinder, so the controlled feature (usually a point or the center of a sphere) may deviate in any direction in space, including axially. That is the key difference from a regular diameter: the cylinder leaves one direction free, the sphere does not.
How to verify
Coordinate measurement, taking points distributed over the spherical surface at several heights and fitting a sphere: this gives the actual diameter and the location of the center, which is what is compared with the spherical tolerance zone. Conventional instruments only give local measurements that guarantee neither the form nor the location of the center. On production spherical seats, spherical gauges or mating parts are used as a quick functional check.
Common mistakes when interpreting it
- Reading the spherical prefix as if it were a cylindrical diameter and evaluating the zone as a cylinder.
- Checking the sphere with two or three diameter measurements and assuming the form is good.
- Forgetting that the spherical zone also controls location along the axis.
How to get it in the workshop / if it comes out of tolerance
It appears on ball seats, ball joints, contact points and dimensions of spherical centers. When you see it in a frame, don't interpret the zone as a cylinder: the feature has to lie within a ball of that diameter, so its location along the axis is controlled too. In machining, a spherical surface is generated with three-axis interpolation or with a form tool, and the error concentrates at the changes of direction.
Error concentrated at the equator or the poles of the sphere: check the interpolation strategy and the tool radius, because the change of axis direction leaves a mark. If the center comes out shifted axially, look at the origin on that axis: it is the error a cylindrical diameter would not have caught and this one does. With a form tool, check profile wear before the parameters.
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